1. Field of the Invention
The present invention relates to a tiltable-body apparatus with a tiltable body which can be reciprocally tilted about a twisting longitudinal axis, such as micro-sensors for sensing mechanical amounts, micro-actuators, and optical micro-scanners, and a method of fabricating the tiltable-body apparatus.
2. Description of the Related Background Art
It is well known that surface forces become more dominant than volume forces as the size of mechanical elements decreases and the influence of friction thus increases in such machines more than in normally-sized machines. Accordingly, in designing micro-machines, it is generally necessary to consider the reduction of the number of sliding portions and rotating portions as much as possible.
A conventional optical scanner with a tiltable body oscillating about a twisting longitudinal axis will be described. FIG. 1 illustrates the optical scanner disclosed in U.S. Pat. No. 4,317,611. FIG. 2 illustrates a disassembled view of this optical scanner to clearly show its internal structure. FIGS. 3 and 4 illustrate cross sections of a silicon thin plate 2020 taken along lines 2003 and 2006 in FIG. 1, respectively.
In the above optical scanner, a recess 2012 is formed in a substrate 2010 of an insulating material. A pair of driver electrodes 2014 and 2016 and a mirror support portion 2032 are provided on the bottom of the recess 2012. A pair of torsion bars 2022 and 2024 and a mirror 2030 are integrally formed in the silicon plate 2020. An upper surface of the mirror 2030 is coated with a highly-reflective material, and the mirror 2030 is rotatably supported by the torsion bars 2022 and 2024. The silicon plate 2020 is disposed above the substrate 2010 with a predetermined distance between the silicon plate 2020 and the driver electrodes 2014 and 2016 being set as illustrated in FIG. 3.
The silicon plate 2020 is electrically grounded. A voltage is alternately applied to each of the driver electrodes 2014 and 2016 to attract the mirror 2030 by an electrostatic force. The mirror 2030 is thus tilted about the longitudinal axis of the torsion bars 2022 and 2024.
The cross section of the torsion bars 2022 and 2024 has a shape of trapezoid as illustrated in FIG. 4. In a microstructure with such torsion bars, however, since the torsion bar is likely to bend in a direction perpendicular to its longitudinal axis, the microstructure can be easily affected by external vibrations and the longitudinal axis of the torsion bar can be easily shifted. Accordingly, it is difficult to attain an accurate driving in such a microstructure.
Therefore, when the above optical scanner is used in an optical scanning type display, its image and spot profile are likely to shift and vary due to the external vibrations. This disadvantage increases when the scanning type display is constructed in a small portable form.
The following structure has been proposed to solve the above-discussed disadvantage of the torsion bar. FIG. 5 illustrates a gimbal plate 2120 for a hard disc head disclosed in “10th International Conference on Solid-State Sensors and Actuators (Transducers '99) pp. 1002–1005”. This gimbal plate 2120 is mounted on a tip portion of a suspension for the hard disc head so that rolling and pitching motions of a magnetic head are flexibly allowed. The gimbal plate includes a support frame 2131 which is rotatably supported by rolling torsion bars 2122 and 2124. There is also arranged inside the support frame 2131 a head support 2130 rotatably supported by pitching torsion bars 2126 and 2128. Twisting axes (indicated by dot-and-dash lines in FIG. 5) of rolling torsion bars 2122 and 2124 and pitching torsion bars 2126 and 2128 are orthogonal to each other, and hence, those torsion bars can achieve rolling and pitching motions of the head support 2130.
FIG. 6 is a cross-sectional view taken along a line 2106 of FIG. 5. As illustrated in FIG. 6, the cross sect ion of each of the torsion bars 2122 and 2124 is T-shaped, and the gimbal plate 2120 has a structure with ribs.
A fabrication method of the above gimbal plate 2120 will be described with reference to FIGS. 7A to 7E. As illustrated in FIG. 7A, initially, a silicon wafer 2191 for molding is perpendicularly etched using an etching method such as ICP-RIE (Inductively Coupled Plasma-Reactive Ion Etching). The silicon wafer 2191 for molding can be re-used. A sacrificial layer 2192 of silicon oxide and phosphosilicate glass is then deposited on the silicon wafer 2191, as illustrated in FIG. 7B. After that, a poly-silicon layer 2193, which is to be the structure of the gimbal plate 2120, is formed as illustrated in FIG. 7C. The poly-silicon 2193 is then patterned as illustrated in FIG. 7D. Finally, the sacrificial layer 2192 is removed, and the poly-silicon layer 2193 is bonded to a patterned pad 2195 with an epoxy resin 2094, as illustrated in FIG. 7E.
The thus-fabricated torsion bar with the T-shaped cross section has the feature that its geometrical moment of inertia I is large while its polar moment of inertia J is relatively small, in contrast to a torsion bar having a circular or rectangular cross section. Therefore, the above torsion bar is relatively easy to twist while hard to bend. That is, this torsion bar has a sufficient compliance in a twisting direction and a high rigidity in a direction perpendicular to the twisting axis.
Further, in the above T-shaped torsion bar, the length for obtaining necessary compliance and permissible twisting angle is small, and hence, the torsion bar can be made compact in size.
Thus, a compact micro-gimbal plate with sufficient compliance in rolling and pitching directions and sufficient rigidity in other directions can be obtained.
However, the above-discussed microstructure has the following disadvantages.
1. In the torsion bar with the T-shaped cross section, a stress concentration is likely to occur at a portion 2150 of FIG. 6 when the torsion bar is twisted. Accordingly, the torsion bar is easy to break.
2. When the torsion bar with the T-shaped cross section is used, a twisting center of the torsion bar deviates from a center of gravity of the tiltable body. This phenomenon will be described with reference to FIGS. 8 and 9. FIG. 8 illustrates a T-shaped torsion bar 2922 one end of which is fixed and the other end of which supports a tiltable body 2930. FIG. 9 illustrates a side of the torsion bar 2922 viewed from a direction of view indicated by an arrow in FIG. 8. As illustrated by arrows in FIG. 9, since the twisting center of the T-shaped torsion bar 2922 deviates from the center of gravity of the tiltable body 2930, a vibratory force occurs in a direction perpendicular to the twisting longitudinal axis when the tiltable body 2930 is tilted. This causes unwanted noises in micro-sensors for mechanical amounts, unnecessary actions in micro-actuators, and deflection shifts of light in micro-optical scanners.
3. Internal loss of poly-silicon is larger than that of single crystal silicon. Accordingly, a mechanical Q-value of the poly-silicon is relatively small. The vibration amplitude cannot hence be increased when the tiltable body is driven by employing its mechanical resonance. Further, its energy efficiency is small since the driving loss is large.